As is known in the art, phased array antennas in general must be calibrated at several levels in order to operate effectively. Since a phased array typically uses many active and passive components to form its aperture illumination (amplitude and phase) state, these components must be properly aligned in order to form a high efficiency radiation beam. While various methods are available to provide the initial calibration state, to correct for variability (recalibration), and to identify failed components for replacement, calibration often can be verified by or controlled through either near field or far field sensing methods. Very large microwave and RF phased array radars have apertures that can have lengths exceeding over 200 wavelengths, for example, in any aperture dimension, making either near or far field sensing expensive and difficult.
In one known technique, individual components are calibrated in the chain extending from the antenna beam port. After calibration in the factory, these components are assembled in the field to produce an initially calibrated aperture. However, this type of calibration has a number of drawbacks, including, for example, reliance on a book keeping system to catalogue the calibrated components in the beam formation chain. Mistakes in keeping track of the components can produce an un-calibrated system. Furthermore, its cost is high due to the need for a precision calibration of a large number of components and cables. In addition, in this method there is no means of verifying the calibrated state, such that whenever a vector state error is produced, which can be due to assembly error, natural component degradation, or error, the error cannot be directly detected.
Another known calibration method that is complicated by large phased arrays relies on a so-called “gold standard.” This calibration approach detects the transmission vector for a series of components usually housed within a common subassembly, such as a TREA (Transmit-Receive Element Assembly). Like the individual calibration approach, this method alone provides no direct sensor to determine whether and to what extent component degradation affects the calibration state.
Some methods rely on near or far field sensors to detect the aperture vector state, both initially, and during the life of the antenna system. Large phased array planar systems impose significant problems for such calibration approaches because of the reliance on near or far field sensors. In the antenna near field, sensors require large and accurate equipment to position a probe antenna in a plane near to and parallel with the fielded antenna aperture plane, or a virtual plane near the aperture. The size and accuracy of known equipment for positioning near field sensors are significant cost factors. Beyond this, known near field sensors require considerable system downtime in order to determine the antenna aperture vector state, which is a prerequisite for calibration and recalibration. Far field calibration sensors alone are a complex and expensive approach for large array calibration because of the significant separation distances involved. For example, a microwave antenna with its longest dimension of 200 wavelengths needs far field sensors separated from the antenna aperture by approximately 80,000 wavelengths. For even larger systems, it quickly becomes very difficult to provide an accurate microwave source at such distances, particularly when the positional accuracy and the implications of fielded antenna beam scanning are also considered.
The calibration methods typically used for phased arrays are clearly disadvantageous for large apertures. In addition, a considerable challenge comes in the form of a digitally beam forming system. As a result, additional microwave connectivity is needed to calibrate the digital channels. Without such calibration, these digital networks may drift, and as a result, produce array and system losses that can degrade the system performance and capabilities.
It will be appreciated that various known RF injection techniques have been used in moderately sized phased array antennas. These include beacon methods, such as the use of aperture peripheral horn antennas, or near field antennas, or the use of array radiating element mutual coupling. However, these methods have significant disadvantages when considered for large phased arrays. For example, the beacon methods require a source antenna in the large array near field, thus causing difficulties in providing physical support while remaining outside of the large array field of view, source physical stability, and limitations on the large array aperture field variation, particularly when frequency scanned elements are used. Mutual coupling methods are also limited when these elements are used, largely because inter-element coupling is not typically sufficient predictable for accurate initial calibration.